1. Field of the Invention
The invention relates to a method for calendering a paper web, which is wound after drying performed at the end of a production section, and then transported to an off-line calender, unwound and passed through the calender.
2. Discussion of Background Information
A method for calendering a paper web is used in conjunction with the manufacture of high-quality papers, such as SC-A papers or SC-A-Plus papers, in which high demands are imposed on the quality of the surface of the web. To ensure adequate printability in the gravure process, certain aspects of paper surface qualities are required.
In order to ensure such quality, off-line calenders having at least ten rollers are used today. Because the off-line calenders operate slower than the paper machines supplying them, a single paper machine often may have two off-line calenders. In contrast to this, SC-B papers are customarily produced on-line, such that the paper web is passed through a 2×2-roller soft calender before being wound. However, to reach the desired surface quality for SC-A or SC-B-Plus papers the above process cannot be utilized.
As the operating speeds of paper machines increase, corresponding changes are needed to the off-line calenders, so the off-line calenders are able to keep up with production. The paper manufacturing industry as a rule, is interested in increasing the speeds of production. However, a consequence of increasing the speed of production is line load and/or surface temperature of the heated rollers have to be increased to meet quality control objectives. This change in the operating parameters may have negative effects on the web, such as a higher calender blackening. Increasing the speed of production could also require operating the calender with more rollers, such as 12 rollers, resulting in increased overall costs of manufacturing.
In extreme cases, the installation of a third off-line calender may be necessary, which again result in increased manufacturing cost. In particular, the increased costs would include the preparation of a standing surface for the third calender.